Asphalt Testing For Precision, Accuracy And Maximum Throughput

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Focus on understanding the key issues associated with proper testing of asphalt binders and solutions to better perform these test.

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Asphalt Testing For Precision, Accuracy And Maximum Throughput

  1. 1. Asphalt Testing for Precision, Accuracy and Maximum Throughput John Casola, Asphalt Market & Products Manager
  2. 2. Asphalt Testing for Precision, Accuracy and Maximum Throughput <ul><li>This seminar will discuss various rheological approaches for asphalt testing to include Superpave PG Grading as well as research related analysis. </li></ul><ul><li>Understanding the best practices for instrumentation operation and how to determine the correct parameters for test set up in order to minimize systematic errors and ensure the data collected are accurate. </li></ul><ul><li>We will cover practical testing limitations to many of the common modes of operation and illuminate the various areas of testing errors. </li></ul><ul><li>We will also discuss the use and benefits of automation to improve performance. </li></ul>
  3. 3. Brief Outline <ul><li>Most common pitfalls </li></ul><ul><li>Sample loading & effects </li></ul><ul><li>Selecting the correct geometry </li></ul><ul><li>Temperature – Temperature – Temperature! </li></ul><ul><li>Operating procedures </li></ul><ul><li>Inertial effects </li></ul><ul><li>Briefly discuss: </li></ul><ul><ul><li>Frequency Sweeps </li></ul></ul><ul><ul><li>Master Curves </li></ul></ul><ul><ul><li>Creep & Creep Recovery </li></ul></ul>
  4. 4. In order to get better data we need to understand some of the most common pitfalls. <ul><li>Geometric Effects </li></ul><ul><ul><li>Edge Effects - </li></ul></ul><ul><ul><ul><li>sample loading – improper trimming, expansion/contraction </li></ul></ul></ul><ul><ul><ul><li>edge failure; reduced sample size as sheared </li></ul></ul></ul><ul><ul><ul><li>sample sag from high temperature or time in the instrument </li></ul></ul></ul><ul><ul><li>Surface area – when to use plates, couette or solids </li></ul></ul>
  5. 5. Sample loading or changing over the time of the test <ul><li>Loading & trimming are a key to good data </li></ul><ul><li>Edge failure due to sag from shearing or long time under test </li></ul><ul><li>Thermal expansion or contraction over a broad range of temperature must be considered </li></ul>
  6. 6. Edge Contributions to the Measurement For 25mm Parallel Plates
  7. 7. Eliminate edge failures for extended testing by using a couette geometry (cup & bob) Master Curve data collection over long times and many temperatures
  8. 8. Glassy state materials are best tested as bars or rods in Torsion
  9. 9. Most common pitfalls. #1 Issue is Temperature <ul><li>Temperature </li></ul><ul><ul><li>Effects of instability </li></ul></ul><ul><ul><li>Large thermal gradients </li></ul></ul><ul><ul><li>Inefficient heat transfer </li></ul></ul><ul><ul><li>Incorrect time to thermal equilibrium </li></ul></ul><ul><ul><li>Do not trust the display of temperature; it is only telling you what the controller is doing. </li></ul></ul><ul><ul><li>You have to look at what the sample is doing to know it is actually stable and accurate. </li></ul></ul>
  10. 10. <ul><li>All Temperature chambers perform differently. </li></ul><ul><li>Many environmental chambers are for generic applications while others are designed for specific applications. </li></ul>Understanding the Importance of Proper Temperature Control
  11. 11. Review of some temperature terms as they relate to asphalt testing <ul><li>Thermal Equilibrium – A condition in which the thermal gradients within the sample are no longer changing. (gradients could be large so long as they are no longer changing) </li></ul><ul><li>Gradients – Temperature variations within the sample; typically from top to bottom and from edge to center. </li></ul><ul><li>Stability - The peak to peak variability measurements of either temperature or modulus over time. </li></ul><ul><li>Thermal Hysteresis - The equilibrium of a sample depends on whether the current temperature (X) was reached from either cooling down to (X) or heating up to (X). </li></ul>
  12. 12. Thermal Conductivity of Water & Air <ul><li>Water ~0.64 </li></ul><ul><li>Air ~0.025 </li></ul><ul><li>Water has about 25 times more thermal conductivity to that of air. </li></ul><ul><li>Water makes a better </li></ul><ul><li>heat transfer medium. </li></ul>
  13. 13. Forced Convection vs. Radiant Plates Heat transfer is a function of velocity of the air flow moving past the heater and in direct contact with the plate Heat transfer is a function of distance from the heater & thermal conductivity of air Plates Gun Heater Heating Elements Dual Tc Plate & Air Cooling gas input Cooling gas input Tc from Plate
  14. 14. <ul><li>Differences between methods of heat transfer medium. </li></ul><ul><ul><li>Dry - Forced Air Convection </li></ul></ul><ul><ul><ul><li>Fast to change temperatures; easily to 60 o C/min </li></ul></ul></ul><ul><ul><ul><li>Heat transfer is a function of the velocity of the air flow </li></ul></ul></ul><ul><ul><ul><li>Needs cooling medium (cold air or LN 2 ) for sub ambient use </li></ul></ul></ul><ul><ul><ul><li>Time to thermal equilibrium can be 15 to 30 minutes </li></ul></ul></ul><ul><ul><ul><li>Thermal gradients can be excessive and difficult to eliminate </li></ul></ul></ul><ul><ul><ul><li>Control of air flow improves gradient errors </li></ul></ul></ul><ul><ul><li>Dry – Radiant Convection </li></ul></ul><ul><ul><ul><li>slower to change temperatures; typically to 15 o C/min </li></ul></ul></ul><ul><ul><ul><li>Heat transfer is a function of the coupling of the air to the sample </li></ul></ul></ul><ul><ul><ul><li>Needs cooling medium (cold air or LN 2 ) for sub ambient use </li></ul></ul></ul><ul><ul><ul><li>Time to thermal equilibrium can be 20 to 40 minutes </li></ul></ul></ul><ul><ul><ul><li>Thermal gradients are excessive and near impossible to eliminate without externally moving the air in the chamber </li></ul></ul></ul>Understanding the Importance of Temperature Control
  15. 15. While you can use Forced Air Ovens for asphalt, accurate calibration of air flow and sample temperature are critical to performance. Heat transfer
  16. 16. Differences between methods of heat transfer medium. <ul><li>Peltier Chambers </li></ul><ul><li>Dual Peltier Element </li></ul>Thermal Conductivity of Air Conduction Temp C Radiant Convection then Conduction Natural convection (chimney) Heat transfer effected by Room Temp, Delta T & Humidity Adapted from a Competitive Patent Temp B Conduction Temp A Tc
  17. 17. Common Methods of Peltier Heating <ul><li>Upper & Lower Heating Chambers </li></ul><ul><ul><li>Dual Independent Heating Elements </li></ul></ul><ul><ul><ul><li>Conduction (lower) & Radiant, Convection, Conduction (upper) </li></ul></ul></ul><ul><ul><ul><ul><li>Lower plate is heated via conduction by the lower element mounted below the lower plate. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Upper plate is heated via Radiant element located in proximity of the upper plate. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Improves time to thermal equilibrium. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Independent heating elements employing different methods of heat transfer make it impossible to stop gradients over a wide range of temperature. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>No mechanism to ensure the temperature at the sample edge is correct. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Gradients vary depending on the direction of heating (hysteresis) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Requires frequent calibration. </li></ul></ul></ul></ul><ul><li>In simple terms: how does one ensure 2 independent heating elements reach the same degree of gradients & equilibrium given all the variables? </li></ul>
  18. 18. Dual Heat Exchanger Peltier w/ Preheated Air Purge Peltier Element Tc PT-100 Dual Heat Exchangers w/ Preheated Purge Patent Pending Purge air
  19. 19. Common Methods of Peltier Heating <ul><li>Upper & Lower Heating Chambers </li></ul><ul><ul><li>Dual Heat Exchangers with preheated air bath </li></ul></ul><ul><ul><ul><li>Conduction (lower) , Radiant (upper) & Forced Convection provides thermal coupling & mixing within the chamber. </li></ul></ul></ul><ul><ul><ul><ul><li>A single heating element is used to heat both upper & lower heat exchangers and to condition an air purge (all to the same temp). </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The single heating element provides reliable calibration over a wide range of temperature. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Stable to variation in room temperature. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Forced convection by the air controls the sample edge temperature. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Significantly improves time to thermal equilibrium. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Significantly reduces thermal gradients. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Virtually eliminates hysteresis. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maintains calibration extremely well. </li></ul></ul></ul></ul>
  20. 20. Differences between methods of heat transfer medium <ul><li>Wet – </li></ul><ul><ul><li>Total Immersion Water Bath </li></ul></ul><ul><ul><ul><li>Slow to change temperatures typically 1 o to 2 o C/min </li></ul></ul></ul><ul><ul><ul><li>Heat transfer is a function of the thermal mass of the water moving past the sample </li></ul></ul></ul><ul><ul><ul><li>Time to thermal equilibrium of the sample is extremely fast </li></ul></ul></ul><ul><ul><ul><li>Thermal gradients are extremely low less then +/-0.01 o C/min </li></ul></ul></ul>
  21. 21. Facts: <ul><li>Asphalt is an extremely good insulator. </li></ul><ul><li>Asphalt does not naturally minimize internal thermal gradients due to its inability to conduct heat efficiently. </li></ul><ul><li>While some thermal chambers react quickly to commanded changes in temperature, the sample of asphalt does not. </li></ul><ul><li>In order to measure accurate asphalt properties, it is important to understand the time required to reach thermal equilibrium </li></ul>
  22. 22. Time to Thermal Equilibrium is Key Depending on the heat transfer medium time to equilibrium can be long From the new AI ‘Best Practices Manual’ Start 10 min equilibration time from this point
  23. 23. To Maximize Efficiency: <ul><li>Time to Thermal Equilibrium. </li></ul><ul><li>Be sure you’ve waited long enough? </li></ul><ul><li>Why wait longer then needed? </li></ul><ul><ul><li>AASHTO requires a 10 minute soak time to ensure everyone gets there. Does it really work? </li></ul></ul><ul><ul><li>For research, just enough is usually more then enough. </li></ul></ul><ul><li>In short, know the required time to reach thermal equilibrium and collect the correct data. </li></ul>
  24. 24. Asphalt Sample loading time to Thermal Equilibrium Statistics should be used to determine the instruments time to thermal equilibrium
  25. 25. Water is the Best Heat Transfer Medium 25 times the thermal conductivity of air Thermal gradients are significant sources of errors. Here you can see a halving of the Modulus just by reducing the thermal gradients
  26. 26. Most common pitfalls. The SOP <ul><li>Bad Operating Procedures </li></ul><ul><ul><li>Using the incorrect time to thermal equilibrium </li></ul></ul><ul><ul><li>Testing with the same sample over too many temperatures in parallel plate </li></ul></ul><ul><ul><li>Not checking for max operating strain first (linearity test) </li></ul></ul><ul><ul><li>Not knowing the max frequency before inertial effects take over. </li></ul></ul>
  27. 27. Standard Operating Procedures <ul><li>By conditioning the sample in the same way from test to test will generally yield the same results which can mask instrument accuracy & precision problems. This just ensures the data is reproducible. </li></ul><ul><li>In the data below, we see that changing the sample conditioning yields </li></ul><ul><li>differing G* data. </li></ul><ul><li>Time at each temp </li></ul><ul><li>was roughly 50min. </li></ul><ul><li>Sample PG 70-22 </li></ul>Pass or Fail? About 10 min
  28. 28. Upper & Lower Heating without an air bath <ul><li>Speed of heating is very good. </li></ul><ul><li>Stability of the </li></ul><ul><li>chamber is excellent. </li></ul><ul><li>Hysteresis, however, </li></ul><ul><li>is still too large. </li></ul><ul><li>Improved thermal </li></ul><ul><li>conductivity is required </li></ul><ul><li>to reduce hysteresis. </li></ul>
  29. 29. Same Data as previous page (scales are set to be the same as the next page)
  30. 30. Addition of an Air Bath to the Chamber (the pre-conditioned purge gas is used to improve thermal conductivity to the edge of the sample)
  31. 31. Thermal Hysteresis is very small ( Δ of 1.5% in G* relates to ~ Δ of 0.075 o C or +/- 0.0375 o C)
  32. 32. Dry chambers are prone to greater errors <ul><li>When checking temperature, make 2 measurements. For the 2 nd measurement, flip your cannon thermistor to understand the gradients from top to bottom of the plates. </li></ul><ul><li>If the errors can’t be corrected with calibration, adjust your temperature for the average of the 2 measurements. </li></ul><ul><li>Don’t use the unit for AMRL reporting </li></ul>
  33. 33. Most common pitfalls: Inertia <ul><li>Inertial Effects </li></ul><ul><ul><li>High frequency errors effect phase angle & viscosity on low modulus (low viscosity) samples. </li></ul></ul><ul><ul><li>High inertia Motor & </li></ul></ul><ul><ul><li>Test Geometry </li></ul></ul><ul><ul><li>on an asphalt at </li></ul></ul><ul><ul><li>high temps (135 o C) </li></ul></ul><ul><ul><li>in oscillation </li></ul></ul><ul><ul><li>Lower viscosity </li></ul></ul><ul><ul><li>exhibits errors at </li></ul></ul><ul><ul><li>lower frequency </li></ul></ul>Limit Limit
  34. 34. To reduce inertial effects reduce mass <ul><li>Inertia for a rotating body is expressed I = 1 / 2 mr 2 </li></ul><ul><ul><li>Where m = mass & r = radius </li></ul></ul><ul><li>Inertia is the sum of all moving parts of the rheometer; plate, chuck, airbearing, position sensor, & motor. </li></ul><ul><ul><li>[ I Total = I 1 + I 2 + I n ] </li></ul></ul><ul><li>To improve instrument results at high frequency: </li></ul><ul><ul><li>Reduce the weight of the testing geometry </li></ul></ul><ul><ul><ul><li>Use lighter metals, Aluminum or Titanium vs. Steel </li></ul></ul></ul><ul><ul><li>Reduce the diameter of the testing geometry </li></ul></ul><ul><ul><ul><li>Smaller diameter plates? </li></ul></ul></ul><ul><ul><ul><li>Smaller bobs? Couette 14mm bob vs. Couette 25mm bob </li></ul></ul></ul><ul><ul><li>Perform Super positioning </li></ul></ul>
  35. 35. Use Time Temperature Super Position; WLF above 0 o C and Arrhenius below
  36. 36. Williams Landel & Ferry TTS Shifted to 135 o C
  37. 37. Creep & Creep Recovery <ul><li>Designed to understand low stress, low rate behavior of materials by measuring over a long period of time. </li></ul><ul><li>Insight to molecular weight (MW) & MW distribution at or near zero shear viscosity. </li></ul><ul><li>Viscous – </li></ul><ul><li>slope relates to 1/n </li></ul><ul><li>Delayed Elastic- </li></ul><ul><li>Elastic- relates to1/G’ </li></ul>Creep Recovery
  38. 38. Multiple Stress Creep Recovery (MSCR) To better understand rutting & effectiveness of modification <ul><li>Applied Stress 100Pa for a 1 second Creep period </li></ul><ul><li>Remove the Stress for a 9 second Recovery period </li></ul><ul><li>Repeated for a total of 10 cycles </li></ul><ul><li>Applied Stress 3200Pa for a 1 second Creep period </li></ul><ul><li>Remove Stress for a 9 second Recovery period </li></ul><ul><li>Repeated for a total of 10 cycles </li></ul><ul><li>Goal is to investigate differences in </li></ul><ul><li>linear & non-linear performance </li></ul>
  39. 39. Example of a Newtonian material Neat binder PG64-28 tested to MSCR at 64 o C
  40. 40. Example of a Visco-Elastic Binder Modified binder PG70-28 tested to MSCR at 70 o C 100Pa 3200Pa
  41. 41. Excessive inertia causes offsets between cycles PG 64-34 MSCR at 100Pa, 64 o C Max Strain ~40%
  42. 42. Without inertial effects; Max Strain ~60% PG 64-28 at 100Pa
  43. 43. The inertial contribution is greater at higher stress PG 64-34 MSCR at 3200Pa, 64 o C Max Strain +1500%
  44. 44. Without inertial effects; Max Strain ~2000% PG 64-28 at 3200Pa
  45. 45. With inertial effects; Max Strain 20% PG 70-22 at 100Pa Creep and Recovery 70-22 SBR 100 Pa 0.00E+00 5.00E+00 1.00E+01 1.50E+01 2.00E+01 2.50E+01 0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02 time s Strain % Series1
  46. 46. Without inertial effects; Max Strain ~30% PG 70-28 at 100Pa
  47. 47. With inertial effects; Max Strain +600% PG 70-22 at 3200Pa
  48. 48. Without inertial effects; Max Strain +700% PG 70-28 at 3200Pa
  49. 49. With inertial effects; Max Strain 25% PG 67-38 at 100Pa
  50. 50. With inertial effects; Max Strain ~1000% PG 67-28 at 3200Pa
  51. 51. In Summary <ul><li>Edge effects can contribute to significant errors. </li></ul><ul><li>Not all temperature controllers work well for asphalt. </li></ul><ul><li>Dry chambers are more prone to errors. </li></ul><ul><li>Excessive thermal hysteresis skews results both higher & lower. </li></ul><ul><li>Flip your Cannon Thermistor to check for gradients. </li></ul><ul><li>Know your instruments performance, determine the time to thermal equilibrium. Most Dry units take longer to get the sample to T E </li></ul><ul><li>Inertial effects in creep recovery can make your data appear to recover less. </li></ul>
  52. 52. Thank you for your interest & time! Contact information <ul><li>Presenter: </li></ul><ul><ul><li>John Casola (US based) </li></ul></ul><ul><ul><li>(+1) 973-740-1543 </li></ul></ul><ul><ul><li>[email_address] </li></ul></ul><ul><ul><li>US Helpdesk: (+1) 508 480 0200 </li></ul></ul><ul><li>Thank you for joining us!!! </li></ul><ul><ul><li>An audio recording of this presentation will be available on the Malvern website soon. Check back with us regularly for future relevant seminars! </li></ul></ul><ul><ul><li>www.malvernevents.com On-Demand Presentations </li></ul></ul>

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